[clangd] Fix highlighting for implicit ObjC property refs

Objective-C lets you use the `self.prop` syntax as sugar for both
`[self prop]` and `[self setProp:]`, but clangd previously did not
provide a semantic token for `prop`.

Now, we provide a semantic token, treating it like a normal property
except it's backed by a `ObjCMethodDecl` instead of a
`ObjCPropertyDecl`.

Differential Revision: https://reviews.llvm.org/D104117